(1) Field of the Invention
This invention relates to a radiographic apparatus for use in medical radiography or industrial nondestructive inspection, for example.
(2) Description of the Related Art
Conventionally, plain radiographic apparatus employing X-ray film and imaging plates have been used in mass medical examinations for tuberculosis and lung cancer. In recent years, apparatus have been developed which enable real-time reading by means of two-dimensional X-ray sensors (hereinafter referred to as flat panel sensors) (e.g. Japanese Patent Publication (Unexamined) S59-211263 and Japanese Patent Publication (Unexamined) H2-164067).
Flat panel sensors include the direct conversion type and indirect conversion type. A flat panel sensor of the direct conversion type has a plurality of switching elements (TFT switches) arranged in a matrix form on a flat panel, with a semiconductor layer superposed thereon. X-ray photons are converted into electric signals in the semiconductor layer to output a fluoroscopic image in electric digital signals. A flat panel sensor of the indirect conversion type has photodiodes and switching elements connected together and arranged in a matrix form, with a layer of scintillator (fluorescent substance) superposed on this photodiode array (semiconductor layer). X rays incident on the scintillator generate optical signals which are converted into electric signals in the photodiodes.
In these flat panel sensors, gate lines common to the respective columns are connected to the switching elements of pixels. A drive signal transmitted to the gate lines turns on the switching elements in the respective columns, whereby charge signals of the pixels are outputted from data lines common to the respective rows. The charge signals outputted are inputted to a signal reading circuit. After a charge-to-voltage transduction and amplification, the signals are put to an analog-to-digital conversion pixel by pixel, and then inputted to an image processing device or the like.
Conventionally, the plain radiographic apparatus employs a separate phototimer in order to avoid underexposure and overexposure. The phototimer has a semiconductor X-ray sensor disposed opposite a front surface of X-ray film (X-ray incidence surface). At an actual radiographing time, the phototimer monitors X-ray doses incident on the X-ray sensor, and a time for terminating the radiography is determined by checking whether an integrated value thereof has exceeded a predetermined value or not.
Such a method, however, poses a problem that the shadow of the phototimer falls on X-ray film. Where the phototimer is formed so thin as not to appear on radiographic images, a required level of X-ray sensitivity cannot be secured.
A proposal has been made recently to achieve an optimal radiography of the same purport with the flat panel sensor developed (e.g. Japanese Patent Publication (Unexamined) H7-72259). The gist of the invention described in this publication lies in frequently taking and adding readings from a predetermined monitor pixel during X-ray irradiation, and blocking X rays when the sum reaches a predetermined quantity.
However, the invention described in Patent Publication H7-72259 has a disadvantage that a reading operation eliminates all data relating to a column that shares a gate line with the monitor pixel. It is conceivable to store the data in a memory to use the data afterward as a radiograph. However, in an actual situation, there occurs a time drift or a lag in reading time, which results in a difference in image quality between the column including the monitor pixel from which data are read frequently, and the other columns of ordinary pixels.